Pneumatic tire with tread including chamfer portions

ABSTRACT

A pneumatic tire having improved wet performance without sacrificing steering stability. A pair of central circumferential main grooves  14  and a pair of side circumferential main grooves  16  are arranged on a tread  12 , to obtain a fundamentally high level of wet performance. A plurality of central land portion row lateral grooves  20  is formed at both sides of a central land portion row  18  to be spaced away from each other substantially at a constant distance, to enhance wet performance. Central land portion row chamfer portions  24  are formed at obtuse angle portions of the central land portion row  18 . The central land portion row chamfer portions  24  drain water between a tread surface and a ground-contact surface into the respective central circumferential main grooves  14  adjacent thereto, whereby wet draining performance is enhanced. Drainage is conducted by the central land portion row chamfer portions  24 , so that it is unnecessary to use multiple central land portion row lateral grooves  20 , thus making it possible to secure land portion rigidity of the central land portion row  18 , and also steering stability.

TECHNICAL FIELD

The present invention relates to a pneumatic tire, and more particularlyto a pneumatic tire in which wet performance is enhanced withoutsacrificing steering stability.

BACKGROUND ART

Circumferential main grooves are formed on a tread of a pneumatic tirein order to obtain wet performance.

It is known that a center rib pattern tire having a center rib thatextends along a tire circumferential direction on the tire equatorialplane is more excellent in steering stability performance than a centergroup pattern tire having grooves in the circumferential direction onthe tire equatorial plane.

When the width of a center rib (i.e., central land portion row) isfurther made larger, although steering stability of the tire isenhanced, conversely, wet performance is deteriorated.

Thus, relating to a passenger car, a light truck or the like, a tirehaving lateral grooves formed on a center rib is generally used in orderto prevent deterioration of wet draining performance (for example,Japanese Patent Application Laid-Open (JP-A) No. 11-91315) However, whena lot of lateral grooves is used for the center rib in order to enhancewet draining performance, a problem is caused that land portion rigidityof the center rib is deteriorated thus deteriorating steering stability.

In other words, in a conventional art, it has been difficult to allowsteering stability and wet performance to coexist.

In view of the aforementioned facts, an object of the present inventionis to provide a pneumatic tire capable of improving wet performancewithout sacrificing steering stability.

DISCLOSURE OF THE INVENTION

The invention in a first aspect is a pneumatic tire in which at least apair of circumferential main grooves extending along a tirecircumferential direction is formed on a tread, the tread is definedinto at least a plurality of land portion rows comprising at least acentral land portion row at the tire equatorial plane side and bilateralland portion rows disposed at tire axial direction outer sides of thecentral land portion row, and a plurality of lateral grooves extendingalong the tire axial direction is formed on the central land portion rowand the bilateral land portion rows, in the tire circumferentialdirection, wherein the lateral grooves which are formed on at least thecentral land portion row are extended from land portion both edges toland portion inner sides by at least 15% or more of the central landportion row tire axial direction width, and the central land portion rowis defined into blocks or false blocks, and the blocks or false blocksform chamfer portions, each having a depth gradually increasing towardthe circumferential main groove and each facing the circumferential maingroove, in the vicinities of the tire circumferential direction one sidecorner portions of the central land portion row, whereby the vicinitiesof both sides in a tire width direction of the central land portion roware made uneven in the tire circumferential direction.

Next, operation and effects of the pneumatic tire according to the firstaspect will be explained.

According to the pneumatic tire of the first aspect, at least a pair ofthe circumferential main grooves is disposed along the tirecircumferential direction of the tread and the tread is defined into atleast the central land portion row at the tire equatorial plane side andthe bilateral land portion rows disposed at the tire axial directionouter sides of the central land portion row, whereby fundamental wetperformance is obtained.

Further, since lateral grooves are formed on the central land portionrow, and extended from land portion both edges to land portion innersides by at least 15% or more of the tire axial direction width of thecentral land portion row, and the central land portion row is definedinto blocks or false blocks, whereby wet performance is enhanced.

Moreover, if the length of each lateral groove is less than 15% of thetire axial direction width of the central land portion row, it does notsuffice for wet performance. Therefore, it is preferred that lateralgrooves are disposed to be spaced away from each other substantially ata constant distance in the tire circumferential direction in order toprevent deviation of draining performance on the tire circumference.

By the way, in order to increase steering stability, it is effective toenlarge the width of the central land portion row. However, due to theexcessive increase of the central land portion row width, a problem iscaused that wet draining performance is deteriorated.

Thus, if lots of lateral grooves (lug grooves) are used in the centralland portion row in order to compensate wet draining performance, landportion rigidity of the central land portion row is deteriorated,resulting in a deterioration of steering stability.

In the pneumatic tire of the present invention, since chamfer portions,each having a depth gradually increasing toward each circumferentialmain groove, are formed in the vicinities of the tire circumferentialdirection one side corner portions of the blocks or false blocks of thecentral land portion row, facing the circumferential main grooves, waterbetween the land portion tread surface and road surface is drainedthrough the chamfer portions into the circumferential main grooves, andwet draining performance can be improved.

In this way, since the chamfer portions drain water, use of lots oflateral grooves becomes unnecessary, high level of land portion rigidityof the central land portion row as well as excellent steering stabilitycan be obtained.

In addition, due to the provision of the chamfer portions at the centralland portion row, the ground-contact area of the central land portionrow is decreased, and high ground-contact pressure can be obtained onthe ground-contact surface.

Accordingly, due to a change of an area of the chamfer portion, anoptimum ground-contact pressure can be obtained.

It is effective to dispose the chamfer portions adjacent to the lateralgrooves in order to increase draining performance. By disposing thechamfer portions adjacent to the lateral grooves, the chamfer portionsare formed in the vicinities of the tire circumferential direction oneside corner portions of blocks and false blocks.

Further, since the chamfer portions are formed, and the transversedirection both side vicinities of the central land portion row are madeuneven in the tire circumferential direction, wet draining performancecan be enhanced without deviation on the tire circumference.

Since the pneumatic tire according to claim 1 is structured as describedabove, the present invention has an excellent effect that wetperformance can be improved without sacrificing steering stability.

The invention in a second aspect is the pneumatic tire wherein thecentral land portion row chamfer portion is formed in the vicinity of anobtuse angled corner portion of the block or the false block as seenfrom a tread plan view of the block or the false block, is formed into asubstantially trapezoid shaped tread plan view configuration whoseupside faces the circumferential main groove side and whose base issubstantially parallel to the tire circumferential direction, and has aplanar shape which is inclined at a constant angle with respect to atread surface.

Next, operation and effects of the pneumatic tire according to thesecond aspect will be explained.

The chamfer portion is formed into a substantially trapezoid shape, theupside of the trapezoid shape is disposed at the circumferential maingroove side, and the area of the chamfer portion becomes smaller at thecircumferential main groove side.

Since the ground-contact area of a portion of the central land portionrow adjacent to the circumferential main groove increases as compared toa case in which the base of the trapezoid shape is disposed at thecircumferential main groove side, unevenness of the ground-contactportion of the central land portion row as a whole can be lessened.

Further, since the base of the trapezoid of the chamfer portion is madesubstantially parallel to the tire circumferential direction, occurrenceof uneven wear can be suppressed (obtaining of resistance to unevenwear).

Moreover, in the present embodiment, “being substantially parallel to atire circumferential direction” includes an inclining angle of less than10° with respect to the tire circumferential direction.

Since the pneumatic tire according to the second aspect is structured asdescribed above, the present invention has an excellent effect thatunevenness of the road-contact portion of the central land portion rowas a whole becomes smaller, whereby occurrence of unevenness can besuppressed.

The invention in a third aspect is the pneumatic tire wherein the tireaxial direction one side lateral groove of the central land portion rowand the tire axial direction other side lateral groove thereof areconnected to each other by the first narrow groove whose width issmaller than those of the lateral grooves.

Next, operation and effects of the pneumatic tire according to the thirdaspect will be explained.

The tire axial direction one side lateral groove of the central landportion row and the tire axial direction other side lateral groovethereof are connected to each other by the first narrow groove whosewidth is smaller than those of the lateral grooves, thus making itpossible to make land portion rigidity of the central land portion roweven on the tire circumference.

Here, it is preferable that the groove width of the first narrow grooveis 2 mm or less.

Since the pneumatic tire according to the third aspect is structured asdescribed above, the present invention has an excellent effect that landportion rigidity of the central land portion row can be made even on thetire circumference.

The invention in a fourth aspect is the pneumatic tire wherein the tireaxial direction width of the central land portion row chamfer portion isset within a range of from 5 to 30% of that of the central land portionrow, and the depth of the tire circumferential main groove side loweredge of the central land portion row chamfer portion is set within arange of from 5 to 50% of that of the tire circumferential main grooveadjacent to the central land portion row chamfer portion.

Next, operation and effects of the pneumatic tire according to thefourth aspect will be explained.

When the tire axial direction width of the central land portion rowchamfer portion is less than 5% of that of the central land portion row,grooved portions (i.e., the area of portions which do not contact theground due to chamfering) become insufficient thus deteriorating wetdraining performance.

On the other hand, when the tire axial direction width of the centralland portion row chamfer portion exceeds 30% of that of the central landportion row, the ground-contact area becomes too lessened thusdeteriorating steering stability.

Accordingly, it is preferable that the tire axial direction width of thecentral land portion row chamfer portion is set within a range of from 5to 30% of that of the central land portion row.

Next, when the depth of the circumferential main groove side lower edgeof the central land portion row chamfer portion is less than 5% of thedepth of the circumferential main groove adjacent to the central landportion row chamfer portion, the volume of the grooved portions as awhole becomes insufficient thus deteriorating wet draining performance.

On the other hand, when the depth of the circumferential main grooveside lower edge of the central land portion row chamfer portion exceeds50% of the depth of the circumferential main groove adjacent to thecentral land portion row chamfer portion, chamfer portion rigidity isdeteriorated thus deteriorating steering stability.

Accordingly, it is preferable that the depth of the tire circumferentialmain groove side lower edge of the central land portion row chamferportion is set within a range of from 5 to 50% of that of the tirecircumferential main groove adjacent to the central land portion rowchamfer portion.

Since the pneumatic tire according to claim 4 is structured as describedabove, the present invention has an excellent effect that wet drainingperformance and steering stability can keep the balance therebetween.

The invention in a fifth aspect is the pneumatic tire wherein a sidewallsurface of the central land portion row non-chamfer portion at thecentral land portion row chamfer portion side is formed at the angle ofsubstantially 90° with respect to the tread surface at a boundaryportion between the central land portion row chamfer portion and thecentral land portion row non-chamfer portion not-including the centralland portion row chamfer portion.

Next, operation and effects of the pneumatic tire according to fifthaspect will be explained.

In the fifth aspect, substantially 90° stands for 90°±10 °.

When the sidewall surface of the central land portion row non-chamferportion at the central land portion row chamfer portion side forms anextremely acute angle (less than 80°) with respect to the tread surface,land portion rigidity of the boundary portion becomes insufficient, anduneven wear easily occurs.

On the other hand, when the sidewall surface of the central land portionrow non-chamfer portion at the central land portion row chamfer portionside forms an extremely obtuse angle (more than 100°) with respect tothe tread surface, edge effects become fragile, and tire performance ona snowy road is deteriorated.

Accordingly, it is preferable that the central land portion rownon-chamfer portion at the central land portion row chamfer portion sideis formed at the angle of substantially 90° with respect to the treadsurface.

Since the pneumatic tire according to the fifth aspect is structured asdescribed above, the present invention has an excellent effect thatuneven wear and tire performance on a snowy road can keep the balancetherebetween.

The invention in a sixth aspect is the pneumatic tire wherein at least aportion of the tire axial direction one side chamfer portion and atleast a portion of the tire axial direction other side chamfer portionare disposed so as to face each other.

Next, operation and effects of the pneumatic tire according to the sixthaspect will be explained.

Since at least a portion of the tire axial direction one side chamferportion and at least a portion of the tire axial direction other sidechamfer portion are disposed so as to face each other, wet drainingperformance and land portion rigidity can be uniform at both sides ofthe central land portion row.

Since the pneumatic tire of the sixth aspect is structured as describedabove, the present invention has an excellent effect that wet drainingperformance and land portion rigidity can be uniformed at the left-handside and the right-hand side of the central land portion row.

The invention in a seventh aspect is the pneumatic tire wherein thecentral land portion row chamfer portion is protruded closer to thecircumferential main groove side adjacent to the central land portionrow chamfer portion than the central land portion row non-chamferportion not including the central land portion row chamfer portionadjacent to the central land portion row chamfer portion in the tirecircumferential direction, and a tire axial direction protruding amountof the central land portion row chamfer portion in reference to thecircumferential main groove side edge of the central land portion rownon-chamfer portion is set within a range of from 2.5 to 40% of a widthof the circumferential main groove adjacent to the central land portionrow chamfer portion.

Next, operation and effects of the pneumatic tire according to theseventh aspect will be explained.

When a land portion is chamfered, ordinarily, land portion rigidity isdeteriorated. However, in the central land portion row, the land portionon which a chamfer portion is formed is protruded closer to thecircumferential main groove side adjacent to the chamfer portion thanthe non-chamfer portion, whereby deterioration of land portion rigiditycan be offset.

Here, when the tire axial direction-protruding amount of the chamferportion is less than 2.5% of the circumferential main groove widthadjacent to the chamfer portion, effects due to protrusion of thechamfer portion are rarely exhibited.

On the other hand, when the tire axial direction-protruding amount ofthe chamfer portion exceeds 40% of the circumferential main groove widthadjacent to the chamfer portion, wet draining performance of thecircumferential main grooves is deteriorated.

Accordingly, it is preferable that the tire axial-direction protrudingamount of the chamfer portion is set within a range of from 2.5 to 40%of the width of the circumferential main groove adjacent to the chamferportion.

Since the pneumatic tire according to the seventh aspect is structuredas described above, deterioration of land portion rigidity due toforming chamfer portions can be offset without deteriorating wetdraining performance.

The invention in an eighth aspect is the pneumatic tire wherein thecentral land portion row chamfer portion is formed only at a portionprotruding closer to the circumferential main groove side than thecentral land portion row non-chamfer portion adjacent to the centralland portion row chamfer portion in the tire circumferential direction.

Next, operation and effects of the pneumatic tire according to theeighth aspect will be explained.

In the pneumatic tire according to the eighth aspect, since the centralland portion row chamfer portion is formed only at a portion protrudingcloser to the circumferential main groove side, steps in the tirecircumferential direction of the central land portion row can be reducedthus making it possible to enhance wear resistance and resistance touneven wear of the central land portion row.

Since the pneumatic tire according to the eighth aspect is structured asdescribed above, excellent effects can be provided in that wearresistance and resistance to uneven wear of the central land portion rowcan be enhanced.

The invention in a ninth aspect is the pneumatic tire wherein the tireaxial direction groove wall of a portion of the central land portion rowprotruding to the circumferential main groove side and the tire axialdirection groove wall of the central land portion row non-chamferportion are connected to a groove bottom portion of the circumferentialmain groove substantially at the same position in the tire axialdirection.

Next, operation and effects of the pneumatic tire according to the ninthaspect will be explained.

In the pneumatic tire according to the ninth aspect, since the tireaxial direction groove wall of a portion of the central land portion rowprotruding to the circumferential main groove side and the tire axialdirection groove wall of the central land portion row non-chamferportion are connected to a groove bottom portion of the circumferentialmain groove substantially at the same position in the tire axialdirection, water in the circumferential main grooves can be flownsmoothly.

Further, in the tire axial direction position as referred herein,“substantially the same position” stands for a position at which thetire axial direction position difference between the groove walls is 1.0mm or less.

Moreover, since the inclination of the tire axial direction groove wallof the non-chamfer portion is gentle than that of the tire axialdirection groove wall of a portion of the central land portion rowprotruding to the circumferential main groove, land portion rigidity ofthe central land portion row can be enhanced.

Since the pneumatic tire according to the ninth aspect is structured asdescribed above, the present invention has an excellent effect thatwater inside the circumferential main grooves can be flown smoothly.

Further, since the pneumatic tire according to the ninth aspect isstructured as described above, the present invention has excellenteffects that land portion rigidity of the bilateral land portion rows isenhanced, and heel-and-toe wear thereof can be suppressed. The presentinvention also has excellent effects that pattern noise can be reducedand wet performance can be improved as compared to a ease where chamferportions are not formed.

The invention in a tenth aspect is the pneumatic tire wherein thebilateral land portion row lateral groove comprises a narrow-widthportion in which a portion of the tire equatorial plane side lateralgroove is formed narrower and a large-width portion in which a remainingportion of the tread edge side lateral groove is formed wider, and aplanar chamfer portion, whose tread plane view is formed into asubstantially rectangular shaped configuration which is longer along thebilateral land portion row lateral groove, is formed in a region wherethe narrow-width portion is formed, and inclined at a constant angle,starting from an imaginary extension line of a tread surface side edgeof the large-width portion toward the circumferential main groove sideadjacent to the bilateral land portion row chamfer portion.

Next, operation and effects of the pneumatic tire according to the tenthaspect will be explained.

Since the width of the bilateral land portion row lateral groove at thetire equatorial plane side is formed narrower, land portion rigidity ofthe bilateral land portion row is enhanced, and block edge motions atboth sides of the narrow-width portion are reduced, whereby heel-and-toewear can be suppressed.

Further, air volume inside the lateral groove having the narrow-widthportion is decreased, and accordingly, pattern noise is decreased.

Moreover, since a planar chamfer portion, whose tread plane view isformed into a substantially rectangular shaped configuration which islonger along the bilateral land portion row lateral groove, is formed inthe narrow-width portion of the bilateral land portion row lateralgroove, and inclined at a constant angle, starting from an imaginaryextension line of a tread surface side edge of the large-width portiontoward the circumferential main groove side adjacent to the bilateralland portion row chamfer portion, the area of the groove is extended,and wet performance is enhanced as compared to a case in which blocksare not chamfered.

Since the pneumatic tire according to the tenth aspect is structured asdescribed above, the present invention has excellent effects that evenperformance can be kept at both sides of the tread, and high level ofwet draining performance can be obtained. Further, the present inventionhas excellent effects that land portion rigidity of the second landportion rows is enhanced and heel-and-toe wear thereof can also besuppressed. The present invention also has excellent effects thatpattern noise can be reduced, and wet performance is enhanced ascompared to a case in which blocks are not chamfered. Further, since thedirection of the bilateral land portion row chamfer portions and thedirection of the second land portion row chamfer portions are opposed toeach other with respect to the tire circumferential direction, deviationof noise and resistance to uneven wear due to a rotational direction ofthe tire can be prevented.

The invention according to an eleventh aspect is the pneumatic tirewherein the central land portion row is disposed on the tire equatorialplane, the second land portion row, which is defined by each of thecircumferential main grooves, is disposed between the central landportion row and each of the bilateral land portion rows, the second landportion row lateral groove comprises a narrow-width portion in which aportion of the tire equatorial plane side lateral groove is formednarrower and a large-width portion in which a remaining portion of thetread edge side lateral groove is formed wider, and a planar chamferportion, whose tread plane view is formed into a substantiallyrectangular shaped configuration which is longer along the second landportion row lateral groove, is formed in a region where the narrow-widthportion of the second land portion row lateral groove is formed, andinclined at a constant angle, starting from an imaginary extension lineof a tread surface side edge of the large-width portion toward thecircumferential main groove side adjacent to the second land portion rowchamfer portion, and the direction of the second land portion rowchamfer portions and that of the bilateral land portion row chamferportions are opposed to each other with respect to the tirecircumferential direction.

Next, operation and effects of the pneumatic tire according to theeleventh aspect will be explained.

Since the central land portion row is disposed on the tire equatorialplane, a uniform arrangement of land portions is enabled at both sidesof the central land portion row, and uniform performance can be providedat both sides thereof.

Since the second land portion row which is defined by each of thecircumferential main grooves is disposed between the central landportion row and each of the bilateral land portion rows, the tread isformed in a pattern of four main grooves, and wet draining performancecan be obtained at high level.

In the second land portion row, since a portion of the tire equatorialplane side lateral groove is formed narrower than the remaining portionthereof, land portion rigidity of the second land portion row can beenhanced, and block edge motions at both sides of the narrow-widthportion can be lessened, and heel-and-toe wear can be suppressed.

Further, air volume inside the lateral groove having the narrow-widthportion is decreased, whereby pattern noise is decreased.

Moreover, since a planar chamfer portion, whose tread plane view isformed into a substantially rectangular shaped configuration which islonger along the second land portion row lateral groove, is formed in aregion where the narrow-width portion of the second land portion rowlateral groove is formed, and inclined at a constant angle, startingfrom an imaginary extension line of a tread surface side edge of thelarge-width portion toward the circumferential main groove side adjacentto the second land portion row chamfer portion, the area of grooves isincreased, and wet performance is further enhanced as compared to a casein which blocks are not chamfered.

Here, when the bilateral land portion row planar chamfer portions andthe second land portion row planar chamfer portions are directed in thesame direction with respect to the tire circumferential direction,leading edges and trailing edges are focused on either the chamferedblock sides or the non-chamfered block sides, and noise and resistanceto uneven wear are deviated due to a rotational direction of the tire.

In the present invention, since the direction of the bilateral landportion row planar chamfer portions and that of the second land portionrow planar chamfer portions are opposed to each other with respect tothe tire circumferential direction, noise and resistance to uneven wearare not deviated due to the tire rotational direction.

Since the pneumatic tire according to the eleventh aspect is structuredas described above, the present invention has excellent effects thatdraining performance through chamfer portions can be obtained, whilepreventing occurrence of heel-and-toe wear or deterioration of patternnoise.

The invention according to a twelfth aspect is the pneumatic tirewherein a lower edge position of the bilateral land portion row chamferportion is set within a range of from 5 to 30% of a depth of thecircumferential main groove adjacent to the bilateral land portion rowchamfer portion, and a tire axial direction width of the bilateral landportion row chamfer portion is set within a range of from 15 to 60% ofthat of the bilateral land portion row width.

Next, operation and effects of the pneumatic tire according to thetwelfth aspect will be explained.

When the lower edge position of the bilateral land portion row chamferportion is less than 5% of the depth of the circumferential main grooveadjacent to the bilateral land portion row chamfer portion, effects dueto chamfering are rarely exhibited.

On the other hand, when the lower edge position of the bilateral landportion row chamfer portion exceeds 30% of the depth of thecircumferential main groove adjacent to the bilateral land portion rowchamfer portion, air volume inside the chamfer portion is increasedthereby allowing the bilateral land portion row chamfer portion tofunction in the same manner as a large-width groove, resulting inoccurrence of heel-and-toe wear or deterioration of pattern noise.

Accordingly, it is preferable that the lower edge position of thebilateral land portion row chamfer portion is set within a range of from5 to 30% of the depth of the circumferential main groove adjacent to thebilateral land portion row chamfer portion.

Next, when the tire axial direction width of the bilateral land portionrow chamfer portion is less than 15% of that of the bilateral landportion row, the area of a portion which is not narrowed increases,whereby heel-and-toe wear easily occurs on the portion.

On the other hand, when the tire axial direction width of the bilateralland portion row chamfer portion exceeds 60% of that of the bilateralland portion row, air volume inside the bilateral land portion rowlateral grooves as a whole becomes insufficient thus deteriorating wetdraining performance.

Accordingly, it is preferable that the tire axial direction width of thebilateral land portion row chamfer portion is set within a range of from15 to 60% of that of the bilateral land portion row width.

Since the pneumatic tire according to the twelfth aspect is structuredas described above, the present invention has an effect that landportion rigidity of blocks or false blocks can be uniformed.

The invention according to a thirteenth aspect is the pneumatic tirewherein the block or the false block of the bilateral land portion rowis defined into a plurality of sub-blocks by a second narrow groovewhose width is smaller than the lateral groove.

Next, operation and effects of the pneumatic tire according to thethirteenth aspect will be explained.

The block or the false block of the bilateral land portion row isdefined into a plurality of sub-blocks by a second narrow groove whosewidth is smaller than the lateral groove, whereby land portion rigidityof the block or the false block can be controlled and land portionrigidity, for example, can be uniformed.

Here, it is preferable that the groove width of the second narrow grooveis 2 mm or less.

Since the pneumatic tire according to the thirteenth aspect isstructured as described above, during a road contacting of the tire,apparent land portion rigidity is increased, and the collapse of blocksor false blocks can be prevented.

The invention in a fourteenth aspect is the pneumatic tire wherein thesecond narrow groove has at least two bent portions at the depthdirection intermediate portions.

Next, operation and effects of the pneumatic tire according to thefourteenth aspect will be explained.

As the tire is rotated, and blocks or false blocks are contacted with aroad surface, upon a receipt of a compressive force in a tire radialdirection, groove walls facing each other of the bent portions are keptin tight contact with each other, and sub-blocks are supported by eachother, whereby apparent land portion rigidity is enhanced, and thecollapse of blocks or false blocks can be prevented.

Since the pneumatic tire according to the fourteenth aspect isstructured as described above, the present invention has effects thatthe direction of an edge effect of the second narrow groove does notchange from the beginning as a new tire product to the end as a worntire, whereby a steady edge effect can be obtained.

The invention in a fifteenth aspect is the pneumatic tire wherein thelengthwise direction of the second narrow groove does not change due toa depth size.

Next, operation and effects of the pneumatic tire according to thefifteenth aspect will he explained.

In accordance with the pneumatic tire of the fifteenth aspect since thelengthwise direction of the second narrow groove which appears on thetread surface does not change as the tire is getting worn, orientationof the edge effect of the second narrow groove does not change from thebeginning as a new tire product to the end as a worn tire.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a tread of a pneumatic tire according to anembodiment of the present invention.

FIG. 2(A) is an enlarged plan view of a central land portion row.

FIG. 2(B) is a cross-sectional view of the central land portion row ofFIG. 2(A) taken along line 2(B)-2(B).

FIG. 2(C) is a cross-sectional view of the central land portion row ofFIG. 2(A) taken along line 2(C)-2(C).

FIG. 3 is a cross-sectional view of a central land portion row accordingto another embodiment of the present invention.

FIG. 4(A) is an enlarged plan view of a second land portion row.

FIG. 4(B) is a cross-sectional view of the second land portion row ofFIG. 4(A) taken along line 4(B)-4(B).

FIG. 5(A) is an enlarged plan view of a bilateral land portion row.

FIG. 5(B) is a cross-sectional view of the bilateral land portion row ofFIG. 5(A) taken along line 5(B)-5(B).

FIG. 5(C) is a cross-sectional view of the bilateral land portion row ofFIG. 5(A) taken along line 5(C)-5(C).

FIG. 6 is a plan view of a tread of a pneumatic tire according toanother embodiment of the present invention.

FIG. 7 is a plan view of a tread of a pneumatic tire according to asecond embodiment of the present invention.

FIG. 8 is a cross-sectional view of the tread shown in FIG. 7 takenalong line A-A.

FIG. 9 is a plan view of a tread of a pneumatic tire according to acomparative example.

FIG. 10 is a plan view of a tread of a pneumatic tire according to aconventional example.

BEST MODES FOR CARRYING OUT THE INVENTION First Embodiment

Hereinafter, with reference to the drawings, the first embodiment of thepresent invention will be described in detail.

As shown in FIG. 1, a tread 12 of a pneumatic tire 10 of the presentembodiment comprises a pair of central circumferential main grooves 14each of which is formed at both sides of the tire equatorial plane CL soas to extend along a tire circumferential direction (the direction ofarrow A (tire rotating direction in the present embodiment), and thedirection of arrow B), and a pair of side circumferential main grooves16 each of which is formed at tire axial direction outer sides of thecentral circumferential main grooves 14 so as to extend along a tirecircumferential direction.

Further, since the pneumatic tire 10 of the present embodiment isstructured in the same manner as in radial tires in general use, adescription of the inner structure thereof will be omitted.

(Central Land Portion Row)

A central land portion row 18, which is defined by a pair of the centralcircumferential main grooves 14, is disposed on the tire equatorialplane CL of the tread 12.

As shown in FIG. 2(A), shorter central land portion row lateral grooves20, each extending inwardly of the land portion from the respectivecentral circumferential main grooves 14, and each terminating inside theland portion, are formed in the vicinities of both edge of the centralland portion row 18.

In the present embodiment, the central land portion row lateral grooves20 at the left-hand side of this figure are inclined with the right-handsides up and are terminated on this side of the tire equatorial planeCL, and similarly, those at a right-hand side of this figure areinclined with the right sides up and are terminated on this side of thetire equatorial plane CL.

The central land portion row lateral grooves 20 at the left-hand side ofthis figure and those at the right-hand side of this figure arepositioned substantially on their extension lines, and are connected toeach other by inclining sipes 22 extending along the extension lines.

A plurality of the central land portion row lateral grooves 20 isarranged at both sides of the central land portion row 18 of the presentembodiment, and the central land portion row 18 is defined into falseblocks. Further, a “false block” referred herein is different from ablock whose entire periphery is surrounded by open-grooves which do notclose during ground contacting of a tire, and refers to a substantiallyblock-shaped block in which a portion of a land portion (30% or less ofa circumferential length of a block) is kept in continuous contact withanother land portion or a substantially block-shaped block in which aportion of a land portion abuts another land portion through a sipe or anarrow groove which partially closes during ground contacting of a tire,and FIG. 2(A) shows a land portion surrounded by a double-chain dot lineas a false block.

In addition, a tire axial direction length L1 of the central landportion row lateral groove 20 of the present embodiment is 27% of a tireaxial direction width W1 of the central land portion row 18. However, atleast 15% of the tire axial direction width W1 suffices for the tireaxial direction length L1, or as shown in FIG. 6, the central landportion row lateral groove 20 can cross the central land portion row 18(100% of the tire axial direction width W1 of the central land portionrow 18 ).

As shown in FIG. 2(A), central land portion row chamfer portions 24 areformed in the vicinities of obtuse angle portions of the central landportion row 18 such that the obtuse angle of the obtuse angle portionsis formed by the central land portion lateral groove 20 and the centralcircumferential main groove 14 connecting thereto and exceeds 90°.

As shown in FIG. 2(B), in the present embodiment, the central landportion row chamfer portion 24 is inclined at a constant angle of θ1 ina tire axial direction, and toward the central circumferential maingroove 14 adjacent thereto, and as shown in FIG. 1, the tread plan viewof the central land portion row chamfer portion 24 is formed into atrapezoid-shaped configuration whose upside faces the centralcircumferential main groove 14 side.

Moreover, a depth gradually increasing substantially in the tire axialdirection and toward the central circumferential main groove 14 adjacentthereto suffices for the depth of the central land portion chamferportion 24, and the direction of the central land portion chamferportion 24 can be slightly inclined (less than 10°) with respect to thetire axial direction.

The central land portion row chamfer portion 24 can be formed into othertread plan view configurations than a trapezoid-shaped configuration.

Here, it is necessary to form the central land portion row chamferportion 24 within the tire axial direction width of the central landportion row lateral groove 20. As shown in FIG. 2(A), it is preferablethat a tire axial direction width W2 is set within a range of from 5 to30% of the tire axial direction width W 1 of the central land portionrow 18.

A chamfer starting position (the base of the trapezoid shape) of thecentral land portion row chamfer portion 24 of the present embodimentcorresponds to a tire axial direction position of a terminal edge 20E ofthe central land portion row lateral groove 20, and the tire axialdirection width W2 is set to 30% of the tire axial direction width W1 ofthe central land portion row 18.

Further, in the present embodiment, although the chamfer startingposition (the base side of the trapezoid shape) of the central landportion row chamfer portion 24 is parallel to the tire circumferentialdirection, the chamfer starting position can be inclined slightly (lessthan 10°) with respect to the tire circumferential direction.

As shown in FIG. 2(B), it is preferable that the depth d1 of the loweredge (the upside of the trapezoid shape) of the central land portion rowchamfer portion 24 at the central circumferential main groove 14 side isset within a range of from 5 to 50% of the depth D1 of the centralcircumferential main groove 14 adjacent to the central land portion rowchamfer portion 24.

In the present embodiment, the depth d1 of the lower edge of the centralland portion row chamfer portion 24 at the central circumferential maingroove 14 side is set to 30% of the depth D1 of the centralcircumferential main groove 14 adjacent to the central land portion rowchamfer portion 24.

When a portion not including the central land portion row chamferportion 24 of the land portion is referred to as a central land portionrow non-chamfer portion 26, as shown in FIG. 2(C), in the presentembodiment, a sidewall surface 26S of the central land portion rownon-chamfer portion 26 at the central land portion chamfer portion 24side is set at 90° with respect to a tread surface.

Moreover, in the present embodiment, although the angle of the sidewallsurface 26S with respect to a tread surface is set at 90°, as shown inFIG. 3, an angle θ2 with respect to the tread surface of the sidewallsurface 26S can be 90°±10°.

In the central land portion row 18 of the present embodiment, as shownin FIG. 2(A), the left-hand side central land portion row chamferportion 24 and the right-hand side central land portion row chamferportion 24 are disposed to face each other at the respective chamferstarting positions (the bases of the trapezoid shapes).

A portion of the central land portion row chamfer portion 24 of thecentral land portion row 18 of the present embodiment is protrudedcloser to the circumferential main groove side adjacent to the centralland portion row chamfer portion than the central land portion rownon-chamfer portion.

It is preferable that a tire axial direction protruding amount t of thecentral land portion row chamfer portion 24 in reference to the centralcircumferential main groove 14 side edge of the central land portion rownon-chamfer portion is set within a range of from 2.5 to 40% of thewidth W3 of the central circumferential main groove 14.

In the present embodiment, the tire axial direction protruding amount tof the central land portion row chamfer portion 24 is set to 20% of thegroove width W3 of the central circumferential main groove 14.

Further, a circumferential sipe 28 extending from the terminal edge 20Eof the central land portion row lateral groove 20 to the central landportion row chamfer portion 24, and a gentle inclining sipe 30 extendingover the tire equatorial plane CL and connecting to the terminal edge ofthe circumferential direction sipe 28 on the other side are formed onthe central land portion row.

(Second Land Portion Rows)

Second land portion rows 32, which are defined by the centralcircumferential main grooves 14 and the side circumferential maingrooves 16, are disposed at the tire axial direction outer sides of thecentral land portion row 18.

As shown in FIG. 4(A), a plurality of second land portion row lateralgrooves 34 crossing in the tire axial direction is formed on the secondland portion rows 32, whereby the second land portion rows 32 aredefined into a plurality of second blocks 36.

A tire equatorial plane CL side portion of each second land portion rowlateral groove 34 is formed narrower than a tire axial direction outerside portion thereof.

Hereinafter, the second land portion row lateral groove 34 of thepresent embodiment comprises a portion referred to as a second landportion row narrow-width portion 34A which is formed narrower and theremaining portion referred to as a second land portion row large-widthportion 34B which is formed wider.

In the present embodiment, a tire circumferential direction side portionof the second block 36 is protruded in order to form the second landportion row narrow-width portion 34A.

Second land portion row chamfer portions 38, each of which is formed ina region where the second land portion row narrow-width portion 34A isformed (a portion protruding from the second block 36), and inclined ata constant angle, starting from an imaginary extension line of the treadsurface side edge of the second land portion row large-width portion 34Bto the second land portion row narrow-width portion 34A, are formed onthe second land portion rows 32 of the present embodiment.

The second land portion row chamfer portion 38 is formed into asubstantially rectangular shaped tread plan view configuration which islonger along the second land portion row lateral groove 34.

As shown in FIG. 4(B), it is preferable that the depth d2 of the loweredge of the second land portion row chamber portion 38 is within a rangeof from 5 to 30% of the depth D1 of the central circumferential maingroove 14 (see FIG. 2(B)) adjacent to the second land portion rowchamber portion 38, and in the present embodiment, the depth d2 is setto 10% of the depth D1 of the central circumferential main groove 14.

As shown in FIG. 4(A), it is preferable that the tire axial directionwidth W4 of the second land portion row chamfer portion 38 is within arange of from 15 to 60% of the tire axial direction width W5 of thesecond block 36, and in the present invention, the tire axial directionwidth W4 is set to 52% of the tire axial direction width W5 of thesecond block 36.

Further, two transverse sipes 40 are formed in the second block 36 so asto cross the second block 36 in the tire axial direction.

(Bilateral Land Portion Rows)

As shown in FIGS. 1 and 5, bilateral land portion rows 42 are disposedat the tire axial direction outer sides of the second land portion rows32.

A plurality of bilateral land portion row lateral grooves 44 crossing inthe tire axial direction is formed in the bilateral land portion row 42,and the bilateral land portion row 42 is defined into a plurality ofshoulder blocks 46.

The width of a portion of the bilateral land portion 42 at the tireequatorial plane CL side is smaller than that of the remaining portionat the tire axial direction outer side thereof.

As shown in FIG. 5, the bilateral land portion row lateral groove 44 ofthe present embodiment comprises a portion referred to as a “bilateralland portion row narrow-width portion 44A which is formed narrower, andthe remaining portion referred to as a “bilateral land portion rowlarge-width portion 44B” which is formed wider.

In the present embodiment, a portion of one side surface in a tirecircumferential direction of the shoulder block 46 is protruded in orderto form the bilateral land portion row narrow-width portion 44A.

Bilateral land portion row chamfer portions 48, each of which is formedin a region where the bilateral land portion row narrow-width portion44A is formed (a portion protruded from the shoulder block 46), andinclined at a constant angle, starting from imaginary extension line ofthe tread surface side edge of the bilateral land portion rowlarge-width portion 44B to the bilateral land portion row narrow-widthportion 44A, are formed on the bilateral land portion rows 42 of thepresent embodiment.

The bilateral land portion row chamfer portion 48 is formed into asubstantially rectangular-shaped tread plane view configuration that islonger along the bilateral land portion row lateral groove 44.

As shown in FIG. 5(B), it is preferable that the depth d3 of the loweredge of the bilateral land portion row chamfer portion 48 is within arange of from 5 to 30% of the groove depth D2 of the sidecircumferential main groove 16, and in the present embodiment, d3 is setto 10% of the depth D2 of the side circumferential main groove 16.

Further, it is preferable that the tire axial direction width W6 of thebilateral land portion row chamfer portion 48 is 15 to 60% of the tireaxial direction width W7 of the shoulder block 46, and in the presentembodiment, W6 is set to 30% of the tire axial direction width W7 of theshoulder block 46.

Moreover, the tire axial direction width W7 of the shoulder block 46referred herein is a size measured from the side circumferential maingroove 16 side edge to the ground-contact edge 12E.

Further, a first axial direction sipe 50 which crosses the shoulderblock 46 in the tire axial direction, a second axial direction 52 whichis formed substantially parallel to the first axial direction sipe 50and which is extended from the side circumferential main groove 16toward the tread edge 12E, terminating near the shoulder block centralportion, and a circumferential short sipe 54 which is extended from theterminal end of the second axial direction sipe 52 in the tirecircumferential direction and is connected to the intermediate portionof the first axial direction sipe 50 at the shoulder block 46 are formedin the shoulder block 46.

Here, the tread edge 12E refers to the tire axial direction outermostend of the ground-contact portion of the pneumatic tire 10 when thepneumatic tire 10 is attached to a standard rim defined in “JATMA YEARBOOK,” (the Japan Automobile Tire Association standards—version 2002),filled with 100% internal pressure of air pressure (the maximum airpressure) corresponding to the maximum load capacity (i.e., bold typedloads of internal pressure-load capacity matching table) in theapplication size/ply rating in JATMA YEAR BOOK, and the maximum loadcapacity is applied thereto.

As shown in FIG. 5(C), the first axial direction sipe 50 and the secondaxial direction sipe 52 are respectively bent at two points of the sipedepth direction intermediate portion as seen from a right cross sectionwith respect to the sipe lengthwise direction.

The first axial direction sipe 50 and the second axial direction sipe 52do not change their lengthwise directions due to the depths thereof.

(Operation)

In the pneumatic tire 10 of the present embodiment, since a pair of thecentral circumferential main grooves 14 and a pair of the sidecircumferential main grooves 16 are arranged on the tread 12 in the tirecircumferential direction, the tread 12 is formed in a pattern of fourmain grooves, and a fundamentally high level of wet performance can beobtained.

Next, the central land portion row 18 will be explained.

The central land portion row 18 is disposed on the tire equatorial planeCL, and the second land portion rows 32 are disposed at outer sides ofthe central land portion row 18, and the bilateral land portions rows 42are also disposed at furthest sides of the second land portion rows 32to provide an uniform land portion arrangement at both sides of the tireequatorial plane CL. Accordingly, uniform performance can be exhibitedat both sides of the tire equatorial plane CL.

A plurality of the central land portion row lateral grooves 20 is formedto be spaced away from each other substantially at a certain distance atboth sides of the central land portion row 18, and the tire axialdirection length L1 of the central land portion row lateral groove 20 isset to 30% of the tire axial direction width W1 of the central landportion row lateral groove 20. Accordingly, wet performance is furtherimproved. Further, since the central land portion row lateral grooves 20are provided so as to be spaced away from each other substantially at acertain distance, deviation of draining performance through the lateralgrooves 20 on the tire circumference can be prevented.

Further, since the central land portion row chamfer portion 24 is formedat an obtuse angle portion of the central land portion row 18 adjacentto the central land portion lateral groove 20, the central land portionrow chamfer portion 24 drains water between a tread surface and a roadsurface into the central circumferential main groove 14 adjacentthereto, whereby wet draining performance can be improved.

Thus, since the central land portion chamfer portion 24 drains water,use of lots of the central land portion row lateral grooves 20 becomesunnecessary, whereby land portion rigidity of the central land portionrow 18 can be obtained, and steering stability can be obtained as well.

By providing the central land portion row chamfer portion 24 at thecentral land portion row 18, a ground-contact area of the central landportion row 18 is decreased, and a ground-contact pressure at high levelcan be obtained. Accordingly, by changing the area of the central landportion row chamfer portion 24, an optimum ground-contact pressure canbe obtained.

Further, since the central land portion row chamfer portion 24 is formedinto a trapezoid shape and the upside of the trapezoid shape is disposedat the central circumferential main groove 14 side, the area of thecentral land portion row chamfer portion 24 is decreased at the centralcircumferential main groove 14 side. As compared to the case in whichthe base of the trapezoid shape is disposed at the centralcircumferential main groove 14 side, a portion of the central landportion row 18 adjacent to the central circumferential main groove 14 isincreased, and degree of deviation of the ground-contact portion of thecentral land portion row 18 as a whole is decreased.

Furthermore, since the base of the trapezoid shape of the central landportion row chamfer portion 24 is made parallel to the tirecircumferential direction, occurrence of uneven wear can be suppressed.

The inclining sipes 22 connecting the central land portion row lateralgrooves 20 at the tire axial direction one side and the central landportion row lateral grooves 20 at the tire axial direction other side,the circumferential direction sipes 28 extending in the tirecircumferential direction from the terminal ends 20E of the central landportion row lateral grooves 20, and similarly, the gentle incliningsipes 30 extending from the terminal ends 20E of the central landportion row lateral grooves 20 over the tire equatorial plane CL, andconnecting the other side circumferential sipes 28 are formed on thecentral land portion row 18. Accordingly, land portion rigidity of thecentral land portion row 18 is uniformed at the tire circumference.

When the tire axial direction width W2 of the central land portion rowchamfer portion 24 is less than 15% of the tire axial direction width W1of the central land portion row 18, grooved portions (the total area ofportions which do not contact the ground due to chamfering) becomeinsufficient thus deteriorating wet draining performance.

On the other hand, when the tire axial direction width W2 of the centralland portion row chamfer portion 24 exceeds 30% of the tire axialdirection width W1 of the central land portion row 18, theground-contact area of the central land portion row 18 becomes too smallthus deteriorating wet draining performance.

Further, when the depth d1 of the lower edge of the central land portionrow chamfer portion 24 is less than 5% of the depth D1 of thecircumferential main groove 14, volumes of the groove portions becomeinsufficient, and wet draining performance is deteriorated.

On the other hand, when the depth d1 of the lower edge of the centralland portion row chamfer portion 24 exceeds 50% of the depth D1 of thecircumferential main groove, land portion rigidity of the central landportion chamfer portion 24 is deteriorated thus deteriorating steeringstability.

Further, since the sidewall surface 26S of the non-chamfer portion 26 isset at 90° with respect to the tread surface, high edge effect requiredfor performance on a snowy road can be obtained, while suppressingoccurrence of uneven wear.

Moreover, when the sidewall surface 26S and the tread surface form anextremely acute angle therebetween (θ2 is less than 80°), land portionrigidity of a boundary portion between the non-chamfer portion 26 andthe central land portion chamfer portion 24 becomes insufficient,whereby uneven wear easily occurs.

On the other hand, the sidewall surface 26S and the tread surface forman extremely obtuse angle therebetween (θ2 exceeds 100°), edge effect isdeteriorated thus deteriorating performance on a snowy road.

In the central land portion row 18 of the present embodiment, since thetire axial direction one side central land portion row chamfer portion24 and the tire axial direction other side central land portion rowchamfer portion 24 are disposed such that portions of the bases of thetrapezoid shapes of the chamfer potions face each other, wet drainingperformance and land portion rigidity are uniform at both sides of thecentral land portion row 18.

In addition, since a portion of the central land portion row chamferportion 24 is protruded closer to the central circumferential maingroove 14 side than the non-chamfer portion 26, deterioration of landportion rigidity due to forming the central land portion row chamferportion 24 can be offset.

Here, when the tire axial direction protruding amount t of the centralland portion row chamfer portion 24 is less than 2.5% of groove width W3of the central circumferential main groove 14, effects due to theprotrusion are rarely exhibited.

On the other hand, when the tire axial direction protruding amount t ofthe central land portion row chamfer portion 24 is less than 2.5% of thewidth W3 of the central circumferential main groove 14, effects due toprotruding the chamfer portion 24 are rarely exhibited.

Next, the second land portion rows 32 will be explained.

In the second land portion row 32, since the second land portion rowlateral groove 34 at the central circumferential main groove 14 side isthe second land portion row narrow-width portion 34A, land portionrigidity of the second land portion row lateral groove 34 (the secondland portion row 32) is improved, and block edge motions at both sidesof the second land portion row narrow-width portion 34A are lessened,and heel-and-toe wear on the second land portion row 32 can be reduced.

Air volume inside the second land portion row lateral groove 34 havingthe second land portion row narrow-width portion 34A is decreased,whereby pattern noise is deteriorated.

In the second land portion row 32, since the second land portion rowchamfer portion 38 is formed in a region where the second land portionrow narrow-width portion 34A is formed, the area of the grooves isincreased, and wet performance is more enhanced than in a case in whichblocks are not chamfered.

Due to providing the transverse sipes 40 at the second land portion row32, land portion rigidity is uniformed.

When the depth d2 of the lower edge of the second land portion rowchamfer portion 38 is less than 5% of the depth D1 of the centralcircumferential main groove 14, effects due to chamfering are rarelyexhibited.

On the other hand, when the depth d2 of the lower edge of the secondland portion row chamfer portion 38 exceeds 30% of the depth D1 of thecentral circumferential main groove 14, air volume of the second landportion row chamfer portion 38 is increased thereby allowing the secondland portion row chamfer portion 38 to function almost in the samemanner as a large-width groove. Accordingly, occurrence of heel-and-toewear or deterioration of pattern noise is induced.

Further, when the tire axial direction width W4 of the second landportion row chamfer portion 38 is less than 15% of the tire axialdirection width W5 of the second block 32, the second land portion rowlarge-width portions 34B becomes relatively larger, and heel-and-toewear easily occurs.

On the other hand, when the tire axial direction width W4 of the secondland portion row chamfer portion 38 exceeds 80% of the tire axialdirection width W5 of the second block 32, the total air volume insidethe second land portion row lateral grooves 34 become insufficient thusdeteriorating wet draining performance.

Next, the bilateral land portions rows 42 will be explained.

In the bilateral land portion row 42, since the bilateral land portionrow lateral groove 44 at the side circumferential main groove 16 side isthe bilateral land portion row narrow-width portion 44A, land portionrigidity of the bilateral land portion row 42 (the shoulder block 46) isenhanced, and motions of block edges at both sides of the bilateral landportion row narrow portion 44A are reduced, whereby heel-and-toe wear inthe bilateral land portion row 42 can be suppressed.

Further, since air volume inside the bilateral land portion row lateralgrooves 44 having the bilateral land portion row narrow-width portion44A is decreased, pattern noise is deteriorated.

Moreover, in the bilateral land portion row 42, since the bilateral landportion row chamfer portion 48 is formed in the region where thebilateral land portion row narrow-width portion 44A is formed, the totalarea of grooves is increased, and wet performance is more enhanced thanin the case in which land portions are not chamfered.

Due to providing the first axial direction sipes 50, the second axialdirection sipes 52, and the circumferential short sipes 54 on thebilateral land portion row 42, land portion rigidity is uniformed.

Here, as the tire is rotated, and blocks or false blocks are contactedwith a road surface, upon a receipt of a compressive force in a tireradial direction, groove walls facing each other are kept in tightcontact with each other between the bent portions of the first axialdirection sipes 50 and the second axial direction sipes 52. Accordingly,apparent land portion rigidity is enhanced, the collapse of the shoulderblocks 46 can be prevented, and uneven wear of the shoulder blocks 46can be suppressed.

Since the first axial direction sipes 50 and the second axial directionsipes 52 do not change their directions in the process during which theshoulder blocks 46 are gradually worn out, direction performances ofedge effects of sipe portions are not changed.

Further, when the depth d3 of the lower edge of the bilateral landportion row chamfer portion 48 is less than 5% of the groove depth D2 ofthe side circumferential main groove 16, effects due to chamfering arerarely exhibited.

On the other hand, when the depth d3 of the lower edge of the bilateralland portion row chamfer portion 48 exceeds 30% of the depth D2 of theside circumferential main groove 16, air volume inside the bilateralland portion row chamfer portion 48 is increased thereby allowing thechamfer portion to function in the same manner as a large-width groove,whereby occurrence of heel-and-toe wear or deterioration of patternnoise is induced.

Further, when the tire axial direction width W6 of the bilateral landportion row chamfer portion 48 is less than 15% of the tire axialdirection width W7 of the bilateral land portion row 42, the bilateralland portion row large-width portion 44B is relatively increased,whereby heel-and-toe wear easily occurs.

On the other hand, when the tire axial direction width W6 of thebilateral land portion row chamfer portion 48 exceeds 80% of the tireaxial direction width W7 of the bilateral land portion row 42, the totalair volume inside the bilateral land portion row lateral grooves 44becomes insufficient, and wet draining performance is deteriorated.

In addition, in the present embodiment, since the direction of thesecond land portion row chamfer portions 38 and that of the bilateralland portion row chamfer portions 48 are opposed to each other withrespect to the tire circumferential direction, noise and resistance touneven wear are not deviated due to the rotation of the tire.

Moreover, in the present embodiment, the tread pattern forms a pointsymmetry. However, the tread pattern can be formed bilaterally symmetricwith the tire equatorial plane CL as a border.

Second Embodiment

A second embodiment of the present invention will be explained withreference to FIGS. 7 and 8. Further, portions identical to those shownin the first embodiment are denoted by the same reference numerals, anddescription thereof will be omitted.

As shown in FIG. 7, in a pneumatic tire 60 of the present embodiment,the size of the central land portion row chamfer portion 24 is differentfrom that in the first embodiment.

The central land portion row chamfer portion 24 of the presentembodiment is formed only at a portion protruding from the edge of thecentral circumferential main groove 14 side central land portion rownon-chamfer portion 26 adjacent thereto in the tire circumferentialdirection.

Further, as shown in FIG. 8, the tire axial direction side groove wall24S of a portion protruding to the central circumferential main groove14 side of the central land portion row 18 and the tire axial directionside groove wall 26S of the central land portion row non-chamfer portion26 are connected to the bottom portion of the central circumferentialmain groove 14 at the same position in the tire axial direction.

(Operation)

In the pneumatic tire 60 of the present embodiment, since the centralland portion row chamfer portion 24 is formed only at a portionprotruding from the central land portion row 18 to the centralcircumferential main groove 14, formation of steps in the tirecircumferential direction of the central land portion row 18 can bereduced, whereby wear resistance and resistance to uneven wear can beenhanced.

Further, in the central land portion row 18, since the tire axialdirection side groove wall 24S of a portion protruding to the centralcircumferential main groove 14 side of the central land portion row 18and the tire axial direction side groove wall 26S of the central landportion row non-chamfer portion 26 are connected to the bottom portionof the central circumferential main groove 14 at the same position inthe tire axial direction. unevenness of the groove walls can besuppressed, and water in grooves can be flown smoothly.

Further, since the inclination of the groove wall 26S of the centralland portion row non-chamfer portion 26 is gentle than that of the wallgroove 24S, land portion rigidity of the central land portion row 18 canbe enhanced.

TEST EXAMPLES

In order to ensure the effects of the present invention, a pneumatictire in Example to which the present invention is applied, a pneumatictire in Comparative Example, and a pneumatic tire in ConventionalExample were prepared, a wet hydroplaning test and a dry steeringstability test were conducted thereon.

Test methods will be explained hereinafter.

Wet hydroplaning test: a test tire (internal pressure: 230 kPa) wasloaded on an actual vehicle (load equivalent to two passengers aboard),a test driver linearly traveled the vehicle on a wet road surface havinga water depth of 5 mm. Evaluation was conducted for feeling evaluationat a speed limit of hydroplaning generation during the vehicletravelling on the aforementioned wet road surface, and expressed by anindex representation with a conventional example as 100. Note that, itis shown that the larger the value of index, the better the tireperformance.

Dry steering stability test: a test tire (size: PSR P235/75R15, treadwidth: 188 mm, and internal pressure: 230 kPa) was loaded on the actualvehicle (load equivalent to two passengers aboard), and a test driverdrove the vehicle by sport driving on a dry circuit course with variousdriving mode. Evaluation was conducted for feeling evaluation, andexpressed by an index representation with a conventional example as 100.Further, it is shown that the larger the value of index, the better thetire performance.

Hereinafter, a description of the test tire will be made.

Example pneumatic tire: the pneumatic tire 10 according to theabove-described embodiments. Groove portion width, groove wall angle andgroove depth are shown in Table 1 as below.

TABLE 1 groove wall angle (to a normal line width of tread surface)groove width (unit: mm) (unit: degree) (unit: mm) centralcircumferential 10 8 10 main groove side circumferential 8 8 10 maingroove central land portion 4 5 9 row lateral groove second land portion4 5 9 row large width portion second land portion 1 0 9 row narrow widthportion bilateral land 4 to 10 5 9 portion row large width portionbilateral land 1 0 9 portion row narrow width portion gentle inclining0.7 0 8 sipe inclining sipe 0.7 0 8 circumferential 0.7 0 3 sipetransverse sipe 0.7 0 8 first axial 0.7 0 8 direction sipe second axial0.7 0 8 direction sipe circumferential 0.7 0 8 short sipe

Comparative Example pneumatic tire: as shown in FIG. 9, ComparativeExample pneumatic tire is structured in the same manner as the Examplepneumatic tire 10 except for the central land portion row chamferportion 24 is not formed at the central land portion row 18.

Conventional Example pneumatic tire: as shown in FIG. 10, centralcircumferential main grooves 114 each extending along the tirecircumferential direction are formed at both sides of the tireequatorial plane CL of a tread 112, and side circumferential maingrooves 116 each extending along the tire circumferential direction areformed at the tire axial direction outer sides of the centralcircumferential main grooves 114.

A central land portion row 118, which is defined by a pair of thecentral circumferential main grooves, is disposed on the tire equatorialplane CL of the tread 112.

Central land portion row lateral grooves 120, each of which is extendedfrom the central circumferential main groove 114 toward the land portioninner side, and is bent and terminated inside the land portions, areformed in the vicinities of both sides of the central land portion row118.

The central land portion row lateral grooves 120 at the left-hand sideand those at the right-hand side thereof in FIG. 10 are connected toeach other by central land portion row first sipes 122 and central landportion row second sipes 124.

Further, central land portion row third sipes 126 are formed at the tirecircumferential direction sides of the central land portion row lateralgrooves 120 of the central land portion row 118.

Second land portion rows 132, which are defined by the centralcircumferential main grooves 114 and the side circumferential maingrooves 116, are disposed at the tire axial direction outer sides of thecentral land portion row 118.

A plurality of second land portion row lateral grooves 134 crossing inthe tire axial direction is formed in the second land portion rows 132,and the second land portion rows 132 are defined into second blocks 136.

A second land portion row first sipe 140 and a second land portion rowsecond sipe 141 are formed in the second land portion row lateral groove134.

Bilateral land portions rows 142 are disposed respectively at the tireaxial direction outer sides of the second land portion rows 132.

A plurality of bilateral land portion row lateral grooves 144 crossingin the tire axial direction is formed in the bilateral land portion rows142, and the bilateral land portion rows 142 are defined into aplurality of shoulder blocks 146.

A bilateral land portion row sipe 150 extending in the tire axialdirection is formed in the shoulder block 146.

Note that, 112E shown in this figure is a tread edge 12E.

Groove portion widths, groove wall angles and groove depths ofConventional

Example pneumatic tire are shown in Table 2 as below.

TABLE 2 groove wall angle (to a normal line width of tread surface)groove width (unit: mm) (unit: degree) (unit: mm) centralcircumferential 9 8 10 main groove side circumferential 6 8 10 maingroove central land portion 4 5 9 row lateral groove second land portion4 5 9 row lateral groove bilateral land 4 to 10 0 8 portion row lateralgroove central land portion 0.7 0 8 row first sipe central land portion0.7 0 8 row second sipe central land portion 0.7 0 8 row third sipesecond land portion 0.7 0 8 row first sipe second land portion 0.7 0 8row second sipe bilateral land 0.7 0 8 portion row sipe

Results of each test are shown in Table 3 as below.

TABLE 3 Conventional Comparative Example Example Example wethydroplaning 100 101 105 performance dry steering stability 100 104 110performance

As a result of the test, it is discovered that dry steering stabilityperformance is enhanced without sacrificing wet hydroplaningperformance.

INDUSTRIAL APPLICABILITY

As described above, the pneumatic tire according to the presentinvention is suitable for the use when the tire is put on to afour-wheel vehicle or the like and is desired to enhance wet performancewithout sacrificing steering stability.

1. A pneumatic tire in which at least a pair of circumferential maingrooves extending along a tire circumferential direction is formed on atread, the tread is defined into at least a plurality of land portionrows comprising at least a central land portion row at the tireequatorial plane side and bilateral land portion rows disposed at tireaxial direction outer sides of the central land portion row, and aplurality of lateral grooves extending along the tire axial direction isformed in the central land portion row and the bilateral land portionrows, in the tire circumferential direction, wherein the lateral grooveswhich are formed on at least the central land portion row are extendedfrom land portion both edges to land portion inner sides by at least 15%or more of the central land portion row tire axial direction width, andthe central land portion row is defined into blocks or false blocks, andthe blocks or the false blocks form chamfer portions, each having adepth gradually increasing toward the circumferential main groove andeach facing the circumferential main groove, in surfaces of the tirecircumferential direction one side corner portions of the central landportion row, whereby the vicinities of both sides in a tire widthdirection of the central land portion row are made uneven in the tirecircumferential direction, and wherein the central land portion rowchamfer portion is formed in the vicinity of an obtuse angled cornerportion of the block or the false block as seen from a tread plan viewof the block or the false block, is formed into a substantiallytrapezoid shaped tread plan view configuration whose upside faces thecircumferential main groove side and whose base is substantiallyparallel to the tire circumferential direction, and has a planar shapewhich is inclined at a constant angle with respect to a tread surface.2. The pneumatic tire of claim 1, wherein the tire axial direction oneside lateral groove of the central land portion row and the tire axialdirection other side lateral groove thereof are connected to each otherby a first narrow groove whose width is smaller than those of thelateral grooves.
 3. The pneumatic tire according to claim 1, wherein atire axial direction width of the central land portion row chamferportion is set within a range of from 5 to 30% of that of the centralland portion row, and a depth of the tire circumferential main grooveside lower edge of the central land portion row chamfer portion is setwithin a range of from 5 to 50% of that of the tire circumferential maingroove adjacent to the central land portion row chamfer portion.
 4. Thepneumatic tire according to claim 1, wherein a sidewall surface of thecentral land portion row non-chamfer portion at the central land portionrow chamfer portion side is formed at the angle of substantially 90°with respect to the tread surface at a boundary portion between thecentral land portion row chamfer portion and the central land portionrow non-chamfer portion not including the central land portion rowchamfer portion.
 5. The pneumatic tire according to claim 1, wherein atleast a portion of the tire axial direction one side central landportion row chamfer portion and at least a portion of the tire axialdirection other side central land portion row chamfer portion aredisposed so as to face each other.
 6. The pneumatic tire according toclaim 1, wherein the central land portion row chamfer portion isprotruded closer to the circumferential main groove side adjacent to thecentral land portion row chamfer portion than the central land portionrow non-chamfer portion not including the central land portion rowchamfer portion adjacent to the central land portion row chamfer portionin the tire circumferential direction, and a tire axial directionprotruding amount of the central land portion row chamfer portion inreference to the circumferential main groove side edge of the centralland portion row non-chamfer portion is set within a range of from 2.5to 40% of a width of the circumferential main groove adjacent to thecentral land portion row chamfer portion.
 7. The pneumatic tireaccording to claim 6, wherein the central land portion row chamferportion is formed only at a portion protruding closer to thecircumferential main groove side than the central land portion rownon-chamfer portion adjacent to the central land portion row chamferportion in the tire circumferential direction.
 8. The pneumatic tireaccording to claim 6, wherein a tire axial direction groove wall of aportion of the central land portion row protruding to thecircum-ferential main groove side and a tire axial direction groove wallof the central land portion row non-chamfer portion are connected to agroove bottom portion of the circumferential main groove substantiallyat the same position in the tire axial direction.
 9. The pneumatic tireaccording to claim 1, wherein the bilateral land portion row lateralgroove comprises a narrow-width portion in which a portion of the tireequatorial plane side lateral groove is formed narrower and alarge-width portion in which a remaining portion of the tread edge sidelateral groove is formed wider, and a planar chamfer portion, whosetread plane view is formed into a substantially rectangular shapedconfiguration which is longer along the bilateral land portion rowlateral groove, is formed in a region where the narrow-width portion isformed, and inclined at a constant angle, starting from an imaginaryextension line of a tread surface side edge of the large-width portiontoward the circumferential main groove side adjacent to the bilateralland portion row chamfer portion.
 10. The pneumatic tire according toclaim 9, wherein the central land portion row is disposed on the tireequatorial plane, a second land portion row, which is defined by each ofthe circumferential main grooves, is disposed between the central landportion row and each of the bilateral land portion rows, a plurality oflateral grooves extending along the tire axial direction is formed inthe second land portion rows in the tire circumferential direction, thesecond land portion row lateral groove comprises a narrow-width portionin which a portion of the tire equatorial plane side lateral groove isformed narrower and a large-width portion in which a remaining portionof the tread edge side lateral groove is formed wider, a planar chamferportion, whose tread plane view is formed into a substantiallyrectangular shaped configuration which is longer along the second landportion row lateral groove, is formed in a region where the narrow-widthportion is formed, and inclined at a constant angle, starting from animaginary extension line of a tread surface side edge of the large-widthportion toward the circumferential main groove side adjacent to thesecond land portion row chamfer portion, and the direction of the secondland portion row chamfer portions and that of the bilateral land portionrow chamfer portions are opposed to each other with respect to the tirecircumferential direction.
 11. The pneumatic tire according to claim 9,wherein a lower edge position of the bilateral land portion row chamferportion is set within a range of from 5 to 30% of a depth of thecircumferential main groove adjacent to the bilateral land portion rowchamfer portion, and a tire axial direction width of the bilateral landportion row chamfer portion is set within a range of from 15 to 60% ofthat of the bilateral land portion row width.
 12. The pneumatic tireaccording to claim 1, wherein the block or the false block of thebilateral land portion row is defined into a plurality of sub-blocks bya second narrow groove whose width is smaller than the lateral groove.13. The pneumatic tire according to claim 12, wherein the second narrowgroove has at least two bent portions at the depth directionintermediate portions.
 14. The pneumatic tire according to claim 13,wherein the lengthwise direction of the second narrow groove does notchange due to a depth size.